Exhibit 99.3

ELECTION FORM AND LETTER OF TRANSMITTAL

Pursuant to the Agreement and Plan of Merger dated August 26, 2026 to which each share of First Illinois Common Stock will be converted into the right to receive either $44.35 in Cash

or 0.5727 of a Share of First Financial Corporation Common Stock

subject in each case, to increase in certain limited circumstances and the proration and procedures described in the Proxy Statement/Prospectus and this Election Form and Letter of Transmittal

Please read and follow the accompanying instructions carefully and deliver to:

THE DEADLINE FOR MAKING AN ELECTION IS 5:00 PM EASTERN TIME ON DECEMBER 16, 2026 (THE “ELECTION DEADLINE”).

THIS ELECTION FORM AND LETTER OF TRANSMITTAL MUST BE COMPLETED, SIGNED AND RECEIVED BY THE EXCHANGE AGENT, ALONG WITH YOUR STOCK CERTIFICATE(S) (IF APPLICABLE), NO LATER THAN THE ELECTION DEADLINE.

PLEASE COMPLETE, SIGN AND MAIL OR DELIVER THIS ELECTION FORM AND LETTER OF TRANSMITTAL, TOGETHER WITH THE CERTIFICATE(S) REPRESENTING YOUR SHARES OF FIRST ILLINOIS CORPORATION COMMON STOCK (IF APPLICABLE), IN THE ENCLOSED ENVELOPE TO THE EXCHANGE AGENT.

If you have questions or need assistance, please contact Paul Nungester, Director of Finance, at: (812) 238-2794 or by email at: pdnungester@first-online.com.

You should complete the Election Form and Letter of Transmittal ONLY if you are a record holder of First Illinois Corporation common stock. If any of your shares of First Illinois Corporation common stock are held in “street name” by a broker, bank or other nominee, please contact your broker, bank or other nominee for instructions on how to make an election for those shares.

Subject to the conditions and limitations set forth in the Merger Agreement, dated as of August 26, 2026, as such agreement may be amended from time to time (the “Merger Agreement”), by and between First Financial Corporation and First Illinois Corporation, pursuant to which First Illinois Corporation will merge with and into First Financial Corporation (the “Merger”), please indicate below your election to receive either Cash Consideration or Stock Consideration (each defined below) in exchange for the shares of First Illinois Corporation common stock you hold of record. By signing and returning this Election Form and Letter of Transmittal together with the requested materials, you understand and acknowledge that, notwithstanding your election below, elections to receive the Cash Consideration and Stock Consideration are subject to the adjustment and proration procedures set forth in the Merger Agreement and described in the proxy statement/prospectus dated and first mailed to holders of First Illinois Corporation common stock on or about November 13, 2026 (the “Proxy Statement/Prospectus”), so that the total number of shares of First Illinois Corporation common stock converted into the Cash Consideration will not exceed 727,377 shares, and the total number of shares of First Illinois Corporation common stock converted into the Stock Consideration will not exceed 1,697,213 shares. You also hereby understand that no assurances can be made (i) that you will receive the amount of Cash Consideration elected, (ii) as to the value of the Cash Consideration relative to the Stock Consideration, (iii) as to the value of the Stock Consideration on the date the Merger is completed and the Stock Consideration is issued, or (iv) as to the value of the consideration you may ultimately receive relative to the value of the First Illinois Corporation common stock exchanged.

(Please check one box only to indicate your election.)

If you fail to submit this Election Form and Letter of Transmittal and any other required documentation referred to herein so that it is received by the Exchange Agent by the Election Deadline, or if none of the boxes above is clearly checked, or if more than one of the boxes above is checked, you will be deemed to have elected to receive the Stock Consideration for your First Illinois Corporation common stock. You should refer to the tax and other matters addressed in the Proxy Statement/Prospectus and to the Merger Agreement before making your election.

The deadline for RECEIPT by the Exchange Agent of this Election Form and Letter of Transmittal and any other documentation referred to herein, including the certificate(s) representing your shares of First Illinois Corporation common stock (if applicable), is 5:00 p.m. Eastern Time on December 16, 2026. You may revoke your election and request the return of your stock certificate(s) by written notice sent to the Exchange Agent at the above address and RECEIVED by the Exchange Agent prior to the Election Deadline. In the event that the Merger Agreement is terminated and the Merger is not consummated, the Exchange Agent will promptly return your stock certificate(s), and you will not receive the Stock Consideration or the Cash Consideration.

All elections must be made on this Election Form and Letter of Transmittal. To make an effective election, you must:

By signing this Election Form and Letter of Transmittal below:

PLEASE ALSO SIGN AND PROVIDE YOUR TAX ID NUMBER IN THE ENCLOSED IRS FORM W-9

(Please read carefully the instructions below)

INSTRUCTIONS

If you have any questions, please contact Paul Nungester, Director of Finance, at: (812) 238-2794 or by email at: pdnungester@first-online.com.

1.             ELECTION DEADLINE AND DELIVERY: This document, or a copy of it, must be properly filled in, dated and signed. First Financial Corporation has the discretion, which it may delegate to Equiniti Trust Company, LLC (the “Exchange Agent”) to determine whether any Election Form and Letter of Transmittal is received on a timely basis and whether an Election Form and Letter of Transmittal has been properly completed. It must be delivered, together with (i) your First Illinois Corporation stock certificate(s) (if you hold certificated shares) and (ii) a properly completed W-9 (See Instruction 13 below), to the Exchange Agent at the appropriate address set forth above. Do not send them to First Illinois Corporation or First Financial Corporation.

You may choose any method to deliver this document, your stock certificate(s) (if you hold certificated shares) and any other required documents; however, you assume all risk of non-delivery. If you choose to use the mail, we recommend that you use registered mail, return receipt requested, and that you properly INSURE all stock certificates. Delivery of stock certificates will be considered effective and risk of loss and title to stock certificates will pass only when those stock certificates are actually received by the Exchange Agent at the appropriate address set forth above.

The Exchange Agent will deliver the shares of First Financial Corporation common stock in book entry form and/or payment checks to which you are entitled, only after:

You will only be entitled to receive the merger consideration in respect of First Illinois Corporation common stock for which you have properly completed this Election Form and Letter of Transmittal to the extent that you continue to hold such shares of record as of immediately prior to the effective time for the Merger. For any election contained herein to be considered, this Election Form and Letter of Transmittal, properly completed and signed, together with the necessary documents, including the certificate(s) representing your shares of First Illinois Corporation common stock (if applicable), MUST BE RECEIVED BY THE EXCHANGE AGENT AT THE ADDRESS ON THE FRONT OF THIS ELECTION FORM AND LETTER OF TRANSMITTAL NO LATER THAN 5:00 P.M. EASTERN TIME ON DECEMBER 16, 2026.

2.             DESCRIPTION OF SHARES: Insert in the box on page 1 of this Election Form and Letter of Transmittal the certificate number(s) of each certificate evidencing First Illinois Corporation common stock that you are surrendering (if you hold certificated shares), the number of shares of First Illinois Corporation common stock you hold to be surrendered herewith (both certificated and book entry shares), and the name(s) and address(es) of the registered owners of such shares. If the space provided is insufficient, attach a separate sheet listing this information.

3              ELECTION OPTIONS: On page 2 of this Election Form and Letter of Transmittal, you may indicate whether you would like to receive, in exchange for your shares of First Illinois Corporation common stock, the Stock Consideration, or the Cash Consideration, or a combination thereof. Mark only one selection. The Merger Agreement provides that the total number of shares of First Illinois Corporation common stock converted into the Cash Consideration will not exceed 727,377 shares, and the total number of shares of First Illinois Corporation common stock converted into the Stock Consideration will not exceed 1,697,213. Therefore, you might receive First Financial Corporation common stock or cash in an amount that is not consistent with your election, depending upon the elections of other First Illinois Corporation stockholders. First Illinois Corporation stockholders that are deemed to have made a non-election, or who check both boxes, will be deemed to have elected the Stock Consideration.

4.             NOMINEES: Any record holder of shares of First Illinois Corporation common stock who is a nominee may submit one or more Election Forms and Letters of Transmittal indicating on the form or forms a combination of elections covering up to the aggregate number of shares of First Illinois Corporation common stock owned by such record holder. However, upon the request of First Financial Corporation, such record holders will be required to certify to the satisfaction of First Financial Corporation that such record holder holds such shares of First Illinois Corporation common stock as nominee for the beneficial owners of such shares.

5.            REVOCATION OR CHANGE OF ELECTION FORM AND LETTER OF TRANSMITTAL: Any Election Form and Letter of Transmittal may be revoked or changed by written notice from the person submitting such form to the Exchange Agent, but to be effective, such notice and any replacement Election Form and Letter of Transmittal must be received by the Exchange Agent at or prior to the Election Deadline. First Financial Corporation will have the discretion, which it may delegate to the Exchange Agent, to determine whether any revocation or change is received on a timely basis and whether any such revocation or change has been properly made.

6.             SIGNATURES. Except as otherwise permitted by this Election Form and Letter of Transmittal, you must sign this document exactly the way your name appears on the face of your First Illinois Corporation stock certificates (if you hold certificated shares). If the shares are owned by two or more persons, each one must sign exactly as his or her name appears on the face of the certificates. If shares are registered in different names on several certificates, it will be necessary to complete, sign and submit as many separate copies of this form as there are different registrations of the certificates. If First Illinois Corporation common stock has been assigned by the registered owner, this document should be signed in exactly the same way as the name of the assignor appearing on the stock certificates or transfer documents. If any shares are registered in different names on several certificates, it will be necessary to complete, sign and submit as many separate Election Forms and Letters of Transmittal as there are different registrations. Election Forms and Letters of Transmittal executed by trustees, executors, administrators, guardians, officers of corporations, or others acting in a fiduciary capacity who are not identified as such in the registration must be accompanied by proper evidence of the signer’s authority to act. The documentary evidence must be in a form that is satisfactory to the Exchange Agent.

7.            LOST CERTIFICATE(S): If the certificate(s) that a registered holder (or transferee) wants to surrender have been lost or destroyed, that holder should promptly contact the transfer agent for First Illinois Corporation and you will be sent additional documentation that you must complete in order to exchange your lost or destroyed certificates. There may be a fee to replace lost certificates. You should contact the transfer agent for First Illinois Corporation as promptly as possible as follows with respect to lost or destroyed certificates: First Illinois Shareholder Servies, Attn: Charlie Osborne, 217-872-7583.

8.             TERMINATION OF MERGER: In the event of termination of the Merger Agreement, the Exchange Agent will promptly return certificates representing shares of First Illinois Corporation common stock, and shall surrender uncertificated shares of First Illinois Corporation common stock held in book entry form, after being notified of such termination by First Financial Corporation or First Illinois Corporation. In such event, shares of First Illinois Corporation common stock held through nominees are expected to be available for sale or transfer promptly following the termination of the Merger Agreement. Certificates representing shares of First Illinois Corporation common stock held directly by First Illinois Corporation stockholders will be returned by registered mail.

9.            EVIDENCE OF SHARES/CHECK ISSUED IN THE SAME NAME: If the shares in book entry form representing First Financial Corporation common stock are to be evidenced and/or the check is to be issued in the same name as the surrendered certificate is registered, this Election Form and Letter of Transmittal should be completed and signed exactly as the surrendered certificate is registered (if you hold certificated shares). Signature guarantees are not required if the certificate(s) surrendered herewith are submitted by the registered owner of such shares who has not completed the section entitled “Special Issuance/Payment Instructions” or are for the account of an Eligible Institution, as defined below. If any of the shares surrendered hereby are owned by two or more joint owners, all such owners must sign this Election Form and Letter of Transmittal exactly as written on the face of the certificate(s).

10.           EVIDENCE OF SHARES/CHECK ISSUED IN DIFFERENT NAME: If the section entitled “Special Issuance/Payment Instructions” is completed, then signatures on this Election Form and Letter of Transmittal must be guaranteed by a firm that is a bank, broker, dealer, credit union, savings association or other entity which is a member in good standing of the Securities Transfer Agents Medallion Program (each an “Eligible Institution”).

11.          SPECIAL ISSUANCE/PAYMENT AND DELIVERY INSTRUCTIONS: Indicate the name and address in which the shares in book entry form representing First Financial Corporation common stock are to be evidenced and/or check is to be sent if different from the name and/or address of the person(s) signing this Election Form and Letter of Transmittal. The stockholder is required to give the social security number or employer identification number of the record owner of the shares. If Special Issuance/Payment Instructions have been completed, the stockholder named therein will be considered the record owner for this purpose.

12.           STOCK TRANSFER TAXES: If any shares of First Financial Corporation common stock are to be evidenced in book entry form or any payment of the cash consideration or cash in lieu of fractional shares is to be made in a name other than that of the registered holder(s) of the surrendered First Illinois Corporation certificate(s), it shall be a condition of such issuance and/or payment that the person requesting such exchange either (i) pay to the Exchange Agent in advance any transfer or other taxes required by reason of the issuance or payment to a person other than the registered holder(s), or (ii) establish to the satisfaction of the Exchange Agent that such tax has been paid or is not applicable.

13.          FEDERAL INCOME TAX WITHHOLDING. Under federal income tax law, the Exchange Agent is required to file a report with the IRS disclosing certain cash payments it makes to holders of First Illinois Corporation common stock. Unless an exemption applies, you must provide the Exchange Agent with your correct tax identification number (your “TIN”) on Form W-9 (or Form W-8 if applicable) to avoid “backup withholding” of federal income tax on any cash received upon the surrender. The TIN for an individual is his or her social security number. An IRS Form W-9 is enclosed with this document.

14.           CONSTRUCTION: All questions with respect to this Election Form and Letter of Transmittal raised by holders of First Illinois Corporation common stock (including, without limitation, questions relating to the time limits or effectiveness of revocation of any election and questions relating to computations as to allocations) will be determined by First Financial Corporation and/or the Exchange Agent, whose determination shall be conclusive and binding. First Financial Corporation shall have the right to reject any and all Election Forms and Letters of Transmittal not in the proper form or to waive any irregularities in any such form, although it does not represent that it will do so. First Financial Corporation and/or the Exchange Agent may, but are not required to, take reasonable action to inform holders of First Illinois Corporation common stock of any defects and may take reasonable action to assist such holders to correct any such defects; however, neither First Financial Corporation nor the Exchange Agent is under any obligation to notify a holder of shares of First Illinois Corporation common stock of any defect in an Election Form and Letter of Transmittal.